Naphthalene crystal growth in a gel

ABSTRACT

A METHOD FOR CRYSTAL GROWTH IN A DIFFUSION MEDIUM IS DISCLOSED WHICH UTILIZES A &#34;CO-SOLUTE EFFECT&#34; TO PRODUCE SINGLE CRYSTALS OF A MATERIAL OF INTEREST. GENERALLY, A COSOLUTE IS A MATERIAL SOLUBLE IN THE CHOSEN SOLVENT MEDIUM AND WHICH INCREASES THE SOLUBILITY IN SAID SOLVENT MEDIUM OF SOME MATERIAL OF INTEREST. TO PRACTICE THE INVENTION AS TAUGHT HEREIN, A CO-SOLUTE IS USED WHICH INCREASES THE SOLUBILITY OF THE MATERIAL OF INTEREST WITH CONCENTRATION OF THE CO-SOLUTE.

United States Patent 3,657,372 NAPHTHALENE CRYSTAL GROWTH IN A GEL JohnC. Murphy, Ellicott City, and Henry A. Kues, Jr., Carney, Md., assignorsto the United States of America as represented by the Secretary of theNavy No Drawing. Filed Mar. 10, 1969, Ser. No. 805,893 Int. Cl. B01d9/02; C07c /24 US. Cl. 260-674 A 1 Claim ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION (A) Field of the invention The presentinvention finds utility in the field of crystallization and, morespecifically relates to the growth of single crystals by diffusion inorganic or inorganic gels.

(B) Description of the prior art A number of patents relating to silicagel processes exist. Crystallization from a silica gel is a well-knownprocedure, having been used for nearly a century.

Growth of single crystals of solid materials requires that theinstantaneous supersaturation of the solid phase relative to thedispersed phase be kept at a minimum. This general condition holds forcrystal growth from a sol-vent medium, from a gas or from a melt.Procedures for single crystal growth differ essentially in the manner inwhich this low supersaturation is maintained. The more obviousdifferences in growth conditions i.e., temperature, pressure, etc.,which distinguish the various methods are determined by the need tosatisfy this general condition. For example, simple growth of crystalsfrom aqueous solution is accomplished either by slow evaporation of thesolvent or by slow cooling or heating of the solution (depending onwhether the solubilitytemperature curve is normal or retrograde). Inboth cases the variation of the external parameter, evaporation rate ortemperature, controls the degree of instantaneous supersaturation andhence the rate of precipitation.

The silica gel method of crystal growth, as an example of the generalproblem of growth in gels, is subject to the above-mentioned condition.To illustrate, a typical prior art silica gel process consists of anaddition of an acid to an aqueous solution of sodium metasilicate (NaSiO -9H O) or ordinary water glass (N320 xSiO III I 0) To this mixtureis added a salt solution containing the X component of the desiredcrystal form MX. On standing, the solution sets to an elastic gel. Afterthe gel has set, a salt solution containing the M component of thedesired crystal form is caused to cover the gel. Crystals of the form MXwill appear in the gel usually within a few hours. Of specificimportance is the fact that the gel is a permeable medium into whichdiffusion can occur at room temperature. The rate of diffusion and hencethe degree of supersaturation is determined by the density of the gel,the normality of the solutions containing 3,657,372 Patented Apr. 18,1972 M and X, and the temperature. Since this diffusion rate can becontrolled, low solubility crystals which normally cannot be obtainedfrom ordinary aqueous solution can be grown.

Solubility considerations prevent growth of compounds such as mercuricsulfides and silver halides using prior art silica gel methods.

The present invention provides a method which enables the growth ofcrystals of these compounds while retaining the simplicity andconvenience of the silica gel method. Since mercuric sulfide and silverhalide crystals are of considerable technological importance, thepresent method marks a significant advance in the field of crystalproduction.

SUMMARY Growth of single crystals of mercuric sulfides and silverhalides is achieved through a modification of the wellknown silica gelprocess. In the above-given description of a typical silica gel process,a solution containing cation M of the desired crystal form MX was addedto a prepared gel. In the present method, the desired compound MX isbrought into solution through the use of a cosolute, that is, a materialwhich increases the solubility of the compound MX in either linear ornon-linear fashion with concentration of the co-solute. The solutioncontaining the desired compound MX is added to the prepared gel. In thecrystal growth operation, the concentration of the combined solution isreduced by diffusion into the gel. As deffusion into the gel of theionic partners, M and X occurs, the compound MX crystallizes due to thereduced solubility of the compound MX in the gel environment. Theconcentration of the co-solute in the gel medium is much less than inthe combined solution and, due to the solubility effect, solvationinfluences are not strong enough to prevent crystallization of thematerial MX.

At the time of this writing, single crystals of mercuric sulfides andsilver halides have been grown by use of the present method utilizing anon-linear solubility dependence. These compounds were previouslyunobtainable through use of aqueous methods. A number of organic andinorganic materials have been grown in aqueous and non-aqueous solutionsusing a linear solubility dependence of the material of interest on theco-solute. One example is a solution of naphthalene in acetone which isadded to an aqueous gel, the naphthalene forming large single crystalsin the gel. The present method, in principle, has general application tocrystal growth of related materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Single crystal growth in adiffusion medium, such as a silica gel, is accomplished by utilizing aco-solute effect. Generally, a co-solute is a material soluble in thesolvent medium in use and which increases the solubility in that mediumof some material of interest. To practice the invention as taughtherein, a co-solute is used which produces a spatial dispersion of thematerial of interest in the diffusion medium. The solubility dispersionis effected by one of at least two mechanisms, viz, (1) a non-lineardependence of the solubility of the material of interest on theconcentration of the co-solute, and (2) a differential rate of diffusionof the co-solute and the material of interest into the diffusion medium.Both mechanisms may be operable in a particular instance. Moregenerally, the operation of the latter mechanism allows crystallizationof compounds having linear solubility dependence on the co-solute.

Crystallization of mercuric sulfides and silver halides is achieved in agel medium by use of a non-linear cosolute effect. The key to the methodis the determination and use of a co-solute which increases thesolubility of the material of interest in a non-linear fashion withconcentration of the co-solute. Increased solubility of the material ofinterest is believed due to complex formation in the solution, or to theformation of a soluble double salt.

Small crystals of cz-HgS and fi-HgS mm. max.) are grown in silica gelfrom Na S solution under varied conditions of pH and temperature. Inthis case, the aqueous solubilities of a-HgS and S-HgS are known to beenhanced by the presence of Na S. Typically, sodium silicate gels ofspecfiic gravity 1.05 in the pH range 5.8-8.5 are formed with nitricacid and allowed to stand overnight. A combined solution of Na s-HgS isadded to the top of the gel. In a matter of a day, crystals appear inthe gel near the interface and in the solution surmounting the gel.Severe gel shrinkage is evident. Dilution of the original combinedsolution by the syneresis liquid from the gel is believed to be theprincipal reason for crystal formation in said solution. Initially, onlycubic, black a-HgS crystals are evident, but on standing for a period oftime, conversion to the red hexagonal B-HgS occurs. The rate ofconversion and the final size of the HgS crystals increase withtemperature in the general range of to 60 C.

Similarly, the solubilities of the various silver halides are known toincrease by many orders of magnitude in concentrated alkali halide andalkaline earth halide solutions as well as in silver complex formerssuch as Na SO' Na S O and others. The various silver halides are grownfrom their respective K, NH Na, 0 and Ca halide solutions. Typical sizesof the crystals are: AgI-8-10 mm. for all co-solutes; AgBr-3 mm. in NHBr (octahedral), -2 mm. in KBr (cubic); AgCl-l mm. in NaCl and CaClsolutions. The procedure employed is as follows. Sodium silicate gels ofspecific gravity 1.03 are formed in the pH range 5.5-8.5 using aceticacid. After gelling, a concentrated solution of the co-solute to beused, for example KI solution, is added to the top of the gel andallowed to stand for times ranging from one to six days. The co-solutesolution is then decanted and the silver halide-co-solute solution, inthe case described AgI in KI solution, is substituted in the place ofthe decanted co-solute solution. Crystals appear within a few hours andgrow to the sizes described previously. This procedure greatlysuppresses the number of nucleation sites observed without pre-doping.In the case of the fi-AgI crystals, in particular, a number of thecrystals had a hexagonal habit with one smooth side, the other sidehaving ridges parallel to the hexagonal sides and radiating from anucleation site in the center of the plate as has previously beendescribed in the art. A number of crystals grew as triangles-1 cm. on aside with ridges from the apices of the triangle to its center. Theremainder of these crystals are perfectly free from other nucleationsites. Optical spectra on these remaining crystals indicate a sharp edgelocated at 2.85 ev., assuming a direct transition. X-ray resultsindicate reasonable crystal perfection.

An example of single crystal growth from a gel medium of a materialhaving linear solubility dependence on a cosolute is illustrated by thegrowth of naphthalene in an aqueous gel where acetone is used as theco-solute. In this particular case, acetone acts not only as theco-solute, but also as the solvent medium for the combined solution.Addition of the naphthalene-acetone solution to an aqueous gel resultsin the formation of naphthalene crystals in the gel. The operablemechanism in this instance is the differential diffusion rate of thenaphthalene and acetone species into the gel. The naphthalene-acetonesystem additionally illustrates the general nature of the presentmethod, i.e., the use of organic solvents and aqueous gels, etc.

The present method is believed to be general and applicable to a largenumber of situations and amenable to various conditions of pH,temperature, gel composition, etc. It is believed apparent that thepractice of the present method is not limited to the particulardescription given herein. Variation in concentrations, densities, andmaterials used are to be expected and permitted within the scope of theappended claim.

What is claimed is:

1. A method for growing single crystals of naphthalene in a sodiumsilicate gel comprising,

preparing in a solvent medium a solution of naphthalene and a co-solute,the co-solute comprising acetone and acting to increase in a linearfashion the solubility of naphthalene in the solvent medium, and

introducing the solution into contacting relation With the gel intowhich the naphthalene exhibits a more rapid rate of diffusion than therate of dilfusion of the co-solute, crystallization of said compoundoccurring in the gel.

References Cited UNITED STATES PATENTS 2,441,572 5/1948 Hirschler et al.2083 10 2,941,018 -6/1960 Foreman 260-674 3,371,036 12/1968 Torgesen23301 OTHER REFERENCES OConnor et al.: Nature, 212, p. 68 (1966).

Henisch et al.: Crystal Growth In Gels, J. Phys. Chem. Solids, pp.493-500 (1965).

Strong: Scientific American, March 1962, pp. -62.

OConnor et al.: J. Crystal Growth, 1 (1967), pp. 327- 28.

Holmes: Formation of Crystals in Gels, J. Phys. Chem. 21, pp. 714-715(1917).

NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US.Cl. X.R. 23-305, 668, 300

